Typical conventional reactive acrylic adhesives utilize well known non-borate free radical initiators. U.S. Pat. No. 4,348,503 discloses acrylic adhesives initiated by a dibasic acid perester and a metal ion. U.S. Pat. No. 4,081,308 discloses an acrylic adhesive initiated with a saccharin salt and α-hydroxysulfone. U.S. Pat. No. 4,331,795 discloses an acrylic adhesive cured with hydroperoxide, thiourea and a metal salt. The non-borate initiated reactions for the above adhesives are inhibited by oxygen.
Organoborane free-radical initiation has been reported in the literature. See, for example, Bawn, C. E. H., Margerison, D. and Richardson, N. M., Proc. Chem. Soc., 1959, 397-398; Furukawa, J., Tsuruta, T., J. Polym. Sci., 1958, 28, 727-729; Dotty, P. M., Fouss, R. M., Mark, H., Overberger, C. G. and Smets, G. J. Polym. Sci. 1958, 33, 502-504. Two mechanisms of the oxygen-organoborane reaction have been postulated. Mirviss, S. B., J. Am. Chem. Soc. 1961, 83, 3051-3056; Davies, A. G., Roberts, B. P., J. Chem. Soc. B, 1969, 311-317; Allies, P. G., Brindly, P. B., J. Chem. Soc. B. 1969, 1126-1131. One mechanism involves the homolytic cleavage of the oxygen-oxygen bond to generate an alkoxy radical and a dialkyl boratoxy radical. The other major pathway is believed to involve homolytic cleavage of the alkyl-oxygen bond to form an alkyl radical and a dialkyl boraperoxy radical. The alkyl radical may react with more oxygen to produce an alkylperoxy radical, which may then react with organoborane to regenerate the alkyl radical and the borane peroxide. Thus one of the advantages in the use of organoborane initiated reactions is that atmospheric oxygen can be used as the source for the co-initiator in the formation of radicals.
Approaches to control the reactivity of organoboranes are known. E. Frankland reported the synthesis of triethylborane and its air-stable complex with a Lewis base, e.g., ammonia. Phil. Trans. Royal Soc. Vol. 152, pp. 167-183 (1863). The air-stable amine complex is believed to slow down the oxidation of organoboranes by blocking the borane open site for oxygen binding, which is the first step in the reaction of organoboranes with oxygen. The initiator can thus be stored in the blocked state and then de-blocked with an appropriate de-blocking or de-complexing agent, such as a stronger Lewis acid.
British Patent Specification No. 1,113,722 entitled “Aerobically Polymerisable Compositions,” published May 15, 1968 discloses a polymerizable composition adapted as a structural adhesive and containing acrylic monomer(s) and an peroxide-activated triaryl, e.g., triphenylborane, complex with hydroxide, ammonia, benzene, or an amine. The polymerization is activated by heating or the addition of an acid. The resulting compositions are reportedly useful as adhesives.
Japanese Patent App. 69-100477 discloses a simple adhesive containing methyl methacrylate, tributylborane, and PMMA for use in the bonding of polyolefins or vinyl polymer articles. Excellent tensile shear strengths of over 1800 p.s.i. were reported. Acrylic adhesives polymerized with tributylborane and other trialkylboranes were reported during the 1970's. (See, U.S. Pat. No. 3,527,737 to Masuhara, et al. and GDR Pat. No. 2,321,215 to Masuhara, et al.)
Two-part adhesives utilizing in one part trialkyl-, triphenyl-, or alkylphenylborane blocked with a primary or secondary amine and in the other part an organic acid or aldehyde are reported in U.S. Pat. No. 5,106,928, Can. Patent 2,061,021, U.S. Pat. No. 5,143,884, U.S. Pat. No. 5,310,835, and U.S. Pat. No. 5,376,746.
U.S. Pat. Nos. 5,539,070, 5,690,780 and 5,691,065 disclose polyoxyalkylenepolyamine-blocked organoborane initiator, with a nitrogen to boron ratio of 1:1 to 1.5:1 useful for adhesives to bond low surface energy materials such as polyolefins and polytetrafluoroethylene.
Organoborane alkoxide complexes are known but there is little reported literature. See, Ludman, C. J.; Waddington, T. C. J. Chem. Soc. A 1966, 1816-1819 and Angew. Chem. Int Ed. Engl. 1972, 11, 48-49. A combination of potassium methoxide with triethyl borane resulted in the isolation of the hygroscopic white solid potassium triethylmethoxy borate(III). Similarly, the synthesis of tetraalkylborates, such as sodium tetraethylborate(III) are known. Honeycutt, Jr,. J. B., Riddle, J. M., J. Am. Chem. Soc. 1961, 83, 369-373. The present inventors have shown that these tetraalkylborates are effective as initiators for acrylic adhesives with or without a deblocking agent. The particular advantage of these initiators over amine-blocked boranes is their tendency to remain colorless after cure. In contrast, amine-blocked borane initiated acrylic adhesives yellow with time.
Hydroxy and/or alkoxy complexing agents for organoborane initiators are described in WO 01/32716, published May 10, 2001. The complexed initiator has the following structure: wherein R1 is alkyl C1-10, R2 and R3 are alkyl or phenyl and Cx is an alkoxide, or hydroxide and the value of v is selected so as to provide an effective ratio of oxygen atoms of the alkoxides and/or hydroxides to boron atoms in the complex. Mixtures of hydroxide complexes with alkoxide complexes are suggested. One alkoxide blocking group is illustrated as((−)O—R4)nM(m+)where R4 is independently selected from hydrogen or an organic group, e.g. alkyl or alkylene, M(m+) is a group IA, IIA, or ammonium countercation, n is an integer >0, and m is an integer >0. An organoborane complex having the following structure follows from this: 
It has been observed that cured polymerizable adhesives initiated with amine-blocked boranes begin to turn yellow due to the presence of the amine group. In contrast, the decomposition products of alkoxy-blocked organoborates (alcohols) and alkyl-blocked organoborates (alkanes) do not undergo the unfortunate yellowing with time. The avoidance of discoloration of the cured bond line is important where a bond line must appear on a visually accessible surface of a bonded article for acceptable aesthetics.
There are numerous considerations in the replacement of mechanically fastened articles with adhesive bonding. It has been observed that conventional alkoxyborates have relatively poor air stability. For example when sodium ethoxotriethylborate(III) is exposed to air for 24 hours, the borate loses 46% of its mass and becomes inactive, unable to initiate the polymerization of methacrylate monomers. Air stability is important for meeting the practical problems in the commercial manufacturing and packaging of formulated borane-containing adhesive systems.
Another requirement for practical application of adhesives systems is the control of open time. Open time refers to the ability of an adhesive to cure providing strong bonds after exposure to air. Amine blocked boranes are known to have open times of a matter of minutes. A blocked boron-based initiator having an open time beyond a few minutes would be of industrial importance.
The inventors have discovered a class of 4-coordinate borates where at least two of the coordinates form part of a ring and have surprisingly good air stability enabling their use in packaged two-part polymerizable adhesives. As illustrated below, internally blocked borates exhibit stability in air of several days and retain an ability to cure methacrylate monomers. Moreover, the internally blocked borate initiators of the present invention remain colorless after curing, whereas conventional amine complexed borates are known to exhibit yellowing after curing.